N. meningitidis has 12 LOS immunotypes. Lacto-N-neotetraose ( LNnT, Galb1-4GlcNAcb1-3Galb1-4Glc), has been reported to be the nonreducing terminal tetrasaccharide in the oligosaccharides of the LOS. The binding of anti-My-28, a mouse monoclonal antibody recognzing LNnT, to N. meningitidis LOS was examined by ELISA and immunoblot assay. Types 2, 3, 4, 5, 8, and 9 exhibited strong binding; types 7 and 10 were intermediate; and types 1, 6, 11, and 12 were negative. If an LOS had large and small size components as revealed by SDS-PAGE, the large one was the active component. Antibody binding to the LOS was inhibited best by LNnT, followed by N-acetyllactosamine. In contrast lacto-N- tetraose (Galb1-3GlcNAcb1-3Galb1-4Glc) did not inhibit. These results suggest that the LNnT sequence exists in 8 of the 12 immunotype LOS. Since LNnT structure has been found in human cells and tissues, the presence of LNnT sequence in the meningococcal LOS structure may play a role in the virulence of Neisseria meningitidis.